Implantable pacemakers and implantable cardioverter defibrillators (ICDs) are available for delivering electrical stimulation therapies to a patient's heart, such as bradycardia pacing, cardiac resynchronization therapy (CRT), anti-tachycardia pacing and cardioversion/defibrillation shocks. Medical device technology advancement has led toward smaller and smaller implantable devices. Recently, this reduction in size has resulted in the introduction of leadless intracardiac pacemakers that can be implanted directly in a heart chamber. One advantage of a leadless intracardiac device is the elimination of the use of transvenous, intracardiac leads, resulting in the elimination of complications due to infection associated with a lead extending from a subcutaneous pacemaker pocket transvenously into the heart, for example. Other complications such as “twiddler's syndrome”, lead fracture or poor connection of the lead to the pacemaker are eliminated as the result of the use of a leadless, intracardiac pacemaker.
Cardiac resynchronization therapy (CRT) is an example of a pacing therapy that includes delivering pacing pulses in a heart chamber at a predetermined time interval after a sensed or paced event in another heart chamber to restore or improve heart chamber synchrony. Improved heart chamber synchrony is expected to alleviate symptoms of heart failure. Achieving a positive clinical benefit from CRT, however, may be dependent on several therapy control parameters, such as the timing intervals used to control pacing pulse delivery, e.g., an atrio-ventricular (AV) interval and/or an inter-ventricular (VV) interval. The AV interval controls the timing of ventricular pacing pulses relative to a preceding atrial depolarization, intrinsic or paced. The VV interval controls the timing of a pacing pulse in one ventricle relative to a paced or intrinsic sensed event in the other ventricle. Pacing may be delivered in the right ventricle (RV) and/or the left ventricle (LV) to restore ventricular synchrony.
Cardiac resynchronization utilizing cardiac ventricular pacing therapy and cardiac pacing devices operate by either delivering pacing stimulus to both ventricles or to one ventricle with the desired result of a more or less simultaneous mechanical contraction and ejection of blood from the ventricles. Triggered pacing systems have been developed for delivery of cardiac synchronization therapy, such as described, for example, in U.S. patent application Ser. No. 14/695,004, to Carney et al. Such triggered pacing systems may include a therapy delivery device, such as a pacing device implanted with the left ventricle, that delivers the ventricular pacing therapy and a sensing device, such as a subcutaneously positionable implantable cardio-defibrillator (ICD), that senses a physiological signal to determine a need for therapy, and generate a control signal passed to a trigger signal emitting device when therapy delivery by the therapy delivery device is required. The trigger signal emitting device emits a trigger signal that is detected by the therapy delivery device, which then delivers at least a portion of a CRT therapy to the patient.
Due to a number of factors associated with a variety of patients, such cardiac pacing systems may not always effectively delivery CRT. For example, factors such as varying capture thresholds, pacing lead and/or electrode migration or dislodgement, time required for appropriate signal processing, confounding conduction delays or conduction blockages, diverse electrode placement locations, inappropriately programmed sensed or paced atrio-ventricular (A-V) or ventriculo-ventricular (V-V) delays, and the like may reduce the effectiveness of CRT.
In either form of CRT delivery, whether fusion-based or the more traditional bi-ventricular stimulation, confirming that pacing stimulus captures the systemic ventricle, which in most cases is the left ventricle (LV), is a very important clinical issue so that the desired benefits of the CRT are in fact delivered to a patient. Ideally, each pacing pulse stimulus delivered to a ventricle evokes a response from the ventricle. The delivery of effective left ventricular pacing helps to ensure that the desired evoked response takes place during delivery of bi-ventricular pacing therapy, and is therefore is an important factor in the delivery of bi-ventricular pacing therapy for cardiac resynchronization therapy (CRT).